Method of producing aluminum alloy sheet for lithographic printing plate

ABSTRACT

A method of producing an aluminum alloy sheet for a lithographic printing plate includes homogenizing an ingot of an aluminum alloy at 500 to 610° C. for one hour or more, the aluminum alloy containing 0.05 to 1.5% of Mg, 0.1 to 0.7% of Fe, 0.03 to 0.15% of Si, 0.0001 to 0.10% of Cu, and 0.0001 to 0.1% of Ti, with the balance being aluminum and unavoidable impurities, subjecting the homogenized product to rough hot rolling, a start temperature of the rough hot rolling being 430 to 500° C. and a finish temperature of the rough hot rolling being 400° C. or more, holding the product subjected to the rough hot rolling for 60 to 300 seconds after completion of the rough hot rolling to recrystallize the surface of the product, subjecting the resulting product to finish hot rolling that is finished at 320 to 370° C., and winding up the resulting product in the shape of a coil to obtain a hot-rolled product having a surface with an average recrystallized grain size in a direction perpendicular to a rolling direction of 50 μm or less. The aluminum alloy may contain 2 to 30 ppm of Pb.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of producing an aluminum alloy sheet for a lithographic printing plate. More particularly, the present invention relates to a method of producing an aluminum alloy sheet for a lithographic printing plate which may be suitably surface-roughened by an electrochemical etching treatment and exhibits high productivity.

2. Description of Related Art

An aluminum alloy sheet is generally used as a support for a lithographic printing plate (including an offset printing plate). An aluminum alloy sheet used for a support is surface-roughened in order to improve adhesion to a photosensitive film and improve water retention in a non-image area. In recent years, a method that roughens the surface of an aluminum alloy sheet used for a support by an electrochemical etching treatment has been increasingly developed due to excellent plate-making applicability (fitness), excellent printing performance, and a continuous treatment capability using a coil material.

In order to improve the plate wear of a printing plate using an aluminum alloy sheet as a support, the printing plate is subjected to exposure and development using a normal method, followed by a heat treatment (burning treatment) at 200 to 290° C. for 3 to 9 minutes to strengthen an image area. Therefore, an aluminum alloy sheet which exhibits heat resistance (burning resistance) has been desired so that the strength of the support does not decrease during the burning treatment. It is also important that streaks or a non-uniform pattern due to a non-uniform structure or a non-recrystallized portion of an aluminum alloy sheet support does not occur when forming a printing plate.

As an aluminum alloy sheet which can be relatively uniformly surface-roughened by electrolysis using an electrochemical etching treatment and satisfies the above-mentioned strength and heat resistance requirements, an A1050 (aluminum purity: 99.5%) equivalent material, a material obtained by adding specific amounts of Mg and Zn to an A1050 equivalent material, or the like has been utilized (see JP-A-2005-15912). It is possible to obtain hundreds of thousands of clear printed matters by appropriately selecting a photosensitive layer applied to the support.

An aluminum alloy material for a lithographic printing plate based on an A1050 equivalent material has been produced by homogenizing an ingot, hot-rolling the homogenized product, cold-rolling the hot-rolled product while performing process annealing to form a recrystallized structure in which recrystallized grains on the surface of the rolled sheet have an average grain size of 40 μm or less, and subjecting the cold-rolled product to secondary cold rolling, thereby ensuring uniform pit formation during an electrochemical etching treatment and preventing streaks when forming a printing plate. However, a decrease in productivity and an increase in production cost inevitably occur due to process annealing. Therefore, an improvement in production method has been desired.

A method that obtains an aluminum alloy sheet for a lithographic printing plate by cold-rolling a hot-rolled product without performing process annealing has been proposed (see JP-A-11-335761). In this method, hot rolling includes rough hot rolling and finish hot rolling. The start temperature of rough hot rolling is set at 450° C. or more. An aluminum alloy is subjected to rough hot rolling at a rolling speed of 50 m/min or more, a rolling reduction of 30 mm or more, or a single-pass rolling reduction rate of 30%. The finish temperature of rough hot rolling is set at 300 to 370° C. The finish temperature of finish hot rolling is set at 280° C. or more. The rolled product is then wound up in the shape of a coil to control the recrystallization state of the surface of the sheet.

SUMMARY OF THE INVENTION

In order to omit process annealing, it is necessary that an aluminum alloy sheet has been recrystallized when wound up in the shape of a coil after finish hot rolling. It is important that recrystallized grains are minute and uniform in the same manner as in a material subjected to process annealing and that the surface of the sheet is uniformly recrystallized.

The inventors of the present invention conducted tests and studies based on the above-mentioned method in order to obtain such a structure. When an area in which recrystallization occurs to only a small extent or an area in which strain is introduced to only a small extent occurs due to microsegregation during casting, coarse recrystallized grains and minute recrystallized grains tend to be formed after hot rolling. Therefore, it is important to form a minute recrystallized structure when rough hot rolling has been completed to obtain a uniform surface, and to apply an appropriate amount of strain during finish hot rolling to obtain a minute and uniform recrystallized structure. The inventors found that it is important to control the start temperature of rough hot rolling, the holding time from the completion of rough hot rolling to finish hot rolling, and the finish temperature of finish hot rolling.

The present invention was conceived as a result of further tests and studies based on the above findings. An object of the present invention is to provide a method of producing an aluminum alloy sheet for a lithographic printing plate which ensures that the surface of the sheet is uniformly recrystallized with minute and uniform recrystallized grains when the sheet is wound up in the shape of a coil after finish hot rolling, enables the sheet to be cold-rolled to a desired thickness without performing process annealing after hot rolling, ensures that pits are uniformly formed during an electrochemical etching treatment and streaks do not occur when forming a printing plate, and enables an improvement in productivity and a reduction in production cost.

A method of producing an aluminum alloy sheet for a lithographic printing plate according to a first aspect of the present invention which achieves the above object comprises homogenizing an ingot of an aluminum alloy at 500 to 610° C. for one hour or more, the aluminum alloy comprising 0.05 to 1.5% of Mg, 0.1 to 0.7% of Fe, 0.03 to 0.15% of Si, 0.0001 to 0.10% of Cu, and 0.0001 to 0.1% of Ti, with the balance being aluminum and unavoidable impurities, subjecting the homogenized product to rough hot rolling, a start temperature of the rough hot rolling being 430 to 500° C. and a finish temperature of the rough hot rolling being 400° C. or more, holding the product subjected to the rough hot rolling for 60 to 300 seconds after completion of the rough hot rolling to recrystallize the surface of the product, subjecting the resulting product to finish hot rolling that is finished at 320 to 370° C., and winding up the resulting product in the shape of a coil to obtain a hot-rolled product having a surface with an average recrystallized grain size in a direction perpendicular to a rolling direction of 50 μm or less.

In the above method of producing an aluminum alloy sheet for a lithographic printing plate, the aluminum alloy may further comprise 2 to 30 ppm of Pb.

The above method of producing an aluminum alloy sheet for a lithographic printing plate may comprise subjecting the hot-rolled product to only cold rolling to obtain a sheet material having a specific thickness.

According to the present invention, a method of producing an aluminum alloy sheet for a lithographic printing plate can be provided which ensures that the surface of the sheet is uniformly recrystallized with minute and uniform recrystallized grains when the sheet is wound up in the shape of a coil after finish hot rolling, enables the sheet to be cold-rolled to a desired thickness without performing process annealing after hot rolling, ensures that pits are uniformly formed during an electrochemical etching treatment and streaks do not occur when forming a printing plate, and enables an improvement in productivity and a reduction in production cost.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

The meanings and the reasons for limitations to the components of the aluminum alloy sheet for a lithographic printing plate according to the present invention are described below. Most Mg is dissolved in aluminum to improve strength and thermal softening resistance (burning resistance). Mg forms magnesium oxide to improve wettability with a treatment liquid during electrolytic surface roughening. This promotes a surface roughening process. The Mg content is preferably 0.05 to 1.5%. If the Mg content is less than 0.05%, Mg may not exhibit a sufficient effect. If the Mg content exceeds 1.5%, the uniformity of pits formed by the surface-roughening treatment may decrease, whereby a non-image area may easily become dirty.

Fe produces Al—Fe intermetallic compounds, and produces Al—Fe—Si intermetallic compounds with Si. These compounds are dispersed to refine the recrystallization structure. These compounds serve as pit formation starting points so that pits are uniformly formed and are finely distributed during an electrolytic treatment. The Fe content is preferably 0.1 to 0.7%. If the Fe content is less than 0.1%, the distribution of the compounds may become non-uniform so that formation of pits may become non-uniform during an electrolytic treatment. If the Fe content exceeds 0.7%, coarse compounds may be produced to decrease the uniformity of the surface-roughened structure.

Si produces Al—Fe—Si intermetallic compounds with Fe. These compounds are dispersed to refine the recrystallization structure. These compounds serve as pit formation starting points so that pits are uniformly formed and are finely distributed during an electrolytic treatment. The Si content is preferably 0.03 to 0.15%. If the Si content is less than 0.03%, the distribution of the compounds may become non-uniform so that pit formation may become non-uniform during an electrolytic treatment. If the Si content exceeds 0.15%, coarse compounds may be produced. Moreover, Si tends to precipitate to decrease the uniformity of the surface-roughened structure.

Cu is easily dissolved in aluminum. When the Cu content is 0.001 to 0.1%, Cu exhibits a pit refinement effect. If the Cu content exceeds 0.1%, pits may become large and non-uniform during an electrolytic treatment.

Ti refines the ingot structure and the crystal grains. As a result, Ti ensures uniform pit formation during an electrolytic treatment to prevent streaks when forming a printing plate. The Ti content is preferably 0.0001 to 0.1%. If the Ti content is less than 0.0001%, Ti may not exhibit a sufficient effect. If the Ti content exceeds 0.1%, coarse Al—Ti compounds may be produced, whereby the surface-roughened structure may become non-uniform. When adding B together with Ti in order to refine the ingot structure, the Ti content is preferably 0.01% or less.

Pb is concentrated in a surface portion and makes pits minute during an electrolytic treatment to improve pit formation uniformity. This enables a desired pit pattern to be obtained. The Pb content is preferably from 2 to 30 ppm. If the Pb content is less than 2 ppm, Pb may not exhibit a sufficient effect. If the Pb content exceeds 30 ppm, the surface-roughened structure may become non-uniform. Pb concentrated in a surface portion improves the uniformity of the surface-roughened structure, and suppresses activation due to magnesium oxide. It is preferable that the Pb concentration in a surface portion up to a depth of 0.2 μm from the uppermost surface be 100 to 400 times the average Pb concentration. If the Pb concentration in the surface portion is less than 100 times the average Pb concentration, activation due to magnesium oxide may not be sufficiently suppressed. If the Pb concentration in the surface portion exceeds 400 times the average Pb concentration, surface dissolution may occur.

The aluminum alloy sheet for a lithographic printing plate according to the present invention exhibits improved electrolytic graining properties by adding one or more of In, Sn, and Ga in an amount of 0.005 to 0.05% in total. Therefore, a desired pit pattern can be obtained with a small amount of electricity. If the total amount of one or more elements selected from In, Sn, and Ga is less than 0.005%, the effect of addition may be insufficient. If the total amount of one or more elements selected from In, Sn, and Ga exceeds 0.05%, the shape of pits may become non-uniform.

The aluminum alloy sheet for a lithographic printing plate according to the present invention is produced by casting an ingot of the aluminum alloy by means of continuous casting or the like, and subjecting the resulting ingot to homogenization, hot rolling, and cold rolling. The present invention is characterized in that hot rolling includes rough hot rolling and finish hot rolling, and recrystallized grains when winding up the aluminum alloy sheet in the shape of a coil after finish hot rolling are controlled by specifying the rolling start temperature, the rolling finish temperature, and the holding time from rough hot rolling to finish hot rolling to obtain a sheet material having a desired thickness by cold rolling without performing process annealing after finish hot rolling.

Specifically, a non-uniform structure which may cause streaks is removed by facing the rolling-side surface of an ingot of an aluminum alloy having the above-described composition. The resulting product is subjected to a homogenization treatment at 500 to 610° C. for one hour or more. The homogenization treatment causes Fe and Si dissolved to supersaturation to uniformly precipitate. As a result, etch pits formed during an electrolytic treatment have a minute circular shape, whereby plate wear is improved. If the homogenization treatment temperature is less than 500° C., precipitation of Fe and Si may be insufficient. As a result, the pit pattern may become non-uniform. If the homogenization treatment temperature exceeds 610° C., the amount of Fe dissolved increases. As a result, the number of minute precipitates which serve as pit formation starting points decreases. If the homogenization treatment time is less than one hour, precipitation of Fe and Si may become insufficient, whereby the pit pattern may become non-uniform.

Hot rolling is normally carried out in a hot rolling line by subjecting the homogenized product to rough hot rolling on a rough rolling stand, transferring the rolled sheet to a finish rolling stand, subjecting the rolled sheet to finish hot rolling on the finish rolling stand, and winding up the hot-rolled sheet in the shape of a coil. In the present invention, rough hot rolling is started at 430 to 500° C. and finished at 400° C. or more. After the completion of rough hot rolling, the product subjected to rough hot rolling is held for 60 to 300 seconds before starting finish hot rolling on the finish rolling stand to recrystallize the surface of the product.

A Pb concentration in a surface portion up to a depth of 0.2 μm from the uppermost surface of 100 to 400 times the average Pb concentration can be achieved by holding the product subjected to rough hot rolling before starting finish hot rolling.

If the start temperature of rough hot rolling is less than 430° C., the number of rolling passes may increase due to an increase in deformation resistance, whereby productivity may decrease. If the start temperature of rough hot rolling exceeds 500° C., coarse recrystallized grains may be produced during rolling, whereby a streak-shaped non-uniform structure may be obtained. If the finish temperature of rough hot rolling is less than 400° C., recrystallization due to holding after rough hot rolling may become insufficient, whereby a uniform surface structure may not be obtained. If the holding time from the completion of rough hot rolling to finish hot rolling is less than 60 seconds, recrystallization may become insufficient, whereby a uniform surface structure may not be obtained. If the holding time exceeds 300 seconds, coarse recrystallized grains may be partially produced due to the growth of recrystallized grains, whereby minute recrystallized grains may not be obtained upon completion of hot rolling.

The product subjected to rough hot rolling is subjected to finish hot rolling. Finish hot rolling is terminated at 320 to 370° C., and the resulting product is wound up in the shape of a coil. If the start temperature of finish hot rolling is less than 400° C., since the finish temperature of finish hot rolling decreases, recrystallization may become insufficient, whereby streaks may occur. If the finish temperature of finish hot rolling is less than 320° C., recrystallization may occur only partially, whereby streaks may occur. If the finish temperature of finish hot rolling exceeds 370° C., recrystallized grains may become large, whereby streaks may occur.

The product subjected to hot rolling is wound up in the shape of a coil to obtain a hot-rolled product having a surface with an average recrystallized grain size in the direction perpendicular to the rolling direction of 50 μm or less. Therefore, a sheet material having a desired thickness can be obtained by cold rolling the resulting product without performing process annealing after finish hot rolling. An improvement in productivity and a reduction in production cost can thus be achieved. The average recrystallized grain size of the surface of the hot-rolled product in the direction perpendicular to the rolling direction is preferably 40 μm or less.

EXAMPLES

The present invention is described below by means of examples and comparison examples to demonstrate the effects of the present invention. Note that the following examples illustrate a preferred embodiment of the present invention. The present invention is not limited to the following examples.

Example 1 and Comparative Example 1

An aluminum alloy having a composition shown in Table 1 was melted and cast. Each rolling-side surface of the resulting ingot was faced by 5 mm to reduce to the thickness of the ingot to 500 mm. The ingot was then subjected to homogenization and hot rolling. The thickness of the aluminum alloy was reduced to 3 mm after finish hot rolling. The aluminum alloy was then wound in the shape of a coil. The hot-rolled product was cold-rolled to a thickness of 0.3 mm without performing process annealing. In Tables 1 and 2, values outside the conditions according to the present invention are underlined.

TABLE 1 Composition (mass %) Alloy Mg Fe Si Cu Ti Pb A 0.34 0.29 0.12 0.0162 0.0295 — B 0.08 0.63 0.10 0.0120 0.0020 — C 0.11 0.20 0.04 0.0003 0.0009 — D 1.47 0.54 0.11 0.0709 0.0009 — E 0.50 0.27 0.10 0.0036 0.0009 — F 0.10 0.35 0.04 0.0003 0.0070 20 G 0.001 0.29 0.05 0.0952 0.0066 — H 2.16 0.19 0.04 0.0256 0.0155 — Note: The Pb content is indicated in ppm.

TABLE 2 Homogenization Rough hot rolling Rough hot rolling Holding Finish hot rolling finish Production Temp. Time start temperature finish temperature time temperature condition (° C.) (h) (° C.) (° C.) (sec) (° C.) a 540 3 460 470 100 345 b 600 2 475 475  80 365 c 510 3.5 430 430 160 340 d 590 2 470 475  80 365 e 530 4 460 465 360 355 f 560 3 450 450  40 340 g 610 3.5 410 390 130 300 h 480 6 460 470 220 330 Note: Holding time: holding time from completion of rough hot rolling to finish hot rolling

A specimen was sampled from the product subjected to finish hot rolling and wound up in the shape of a coil. The average recrystallized grain size of the surface of the specimen in the direction perpendicular to the rolling direction was measured by the following method. The results are shown in Table 3.

-   Measurement of average recrystallized grain size: After degreasing     and washing the surface of the specimen, the surface of the specimen     was mirror-polished and then anodized using a Parker's reagent. The     crystal grains were observed in a polarization mode of an optical     microscope, and the crystal grain size in the direction     perpendicular to the rolling direction was determined using an     intercept method.

The presence or absence of a non-uniform pattern and streaks on the cold-rolled product was observed, and an unetched area and etch pit uniformity were evaluated by the following methods. The results are shown in Table 3.

The cold-rolled product was subjected to degreasing (solution: 5% sodium hydroxide, temperature: 60° C., time: 10 seconds), neutralization (solution: 10% nitric acid, temperature: 20° C., time: 30 seconds), an alternating-current electrolytic surface-roughening treatment (solution: 2.0% hydrochloric acid, temperature: 25° C., frequency: 50 Hz, current density: 60 A/dm², time: 20 seconds), desmut process (solution: 5% sodium hydroxide, temperature: 60° C., time: 5 seconds), and an anodizing process (solution: 30% sulfuric acid, temperature: 20° C., time: 60 seconds). The product was then washed with water, dried, and cut to a specific size to prepare a specimen.

The presence or absence of a non-uniform pattern and streaks of each specimen was observed. The surface of the specimen was observed using a scanning electron microscope (SEM) at a magnification of 500. The surface of the specimen was photographed so that the field of view was 0.04 mm². An unetched area and etch pit uniformity were evaluated based on the resulting photograph.

-   Presence or absence of non-uniform pattern: A case where a     non-uniform pattern was observed on the surface of the specimen with     the naked eye was evaluated as “Bad”, and a case where a non-uniform     pattern was not observed was evaluated as “Good”. -   Presence or absence of streaks: A case where streaks were observed     on the surface of the specimen with the naked eye was evaluated as     “Bad”, and a case where streaks were not observed was evaluated as     “Good”. -   Evaluation of unetched area: A case where the percentage of an     unetched area exceeded 20% was evaluated as “Bad”, and a case where     the percentage of an unetched area was 20% or less was evaluated as     “Good”. -   Evaluation of etch pit uniformity: A case where the area ratio of     large pits with a circle equivalent diameter exceeding 10 μm was     more than 10% with respect to all pits was evaluated as “Bad”, and a     case where the area ratio was 10% or less was evaluated as “Good”.

TABLE 3 Production Average grain Unetched Pit Non-uniform Specimen Alloy condition size (μm) area uniformity pattern Streaks 1 A a 38 Good Good Good Good 2 B a 40 Good Good Good Good 3 C b 43 Good Good Good Good 4 D c 49 Good Good Good Good 5 E a 48 Good Good Good Good 6 F d 43 Good Good Good Good 7 G b 41 Bad Bad Good Good 8 H c 39 Bad Bad Good Good 9 A e 82 Good Good Bad Bad 10 A f 72 Good Good Bad Bad 11 A g Non- Good Bad Bad Bad recrystallized 12 A h 105 Bad Bad Good Good Note: The average recrystallized grain size refers to an average recrystallized grain size of the surface of the hot-rolled product in the direction perpendicular to the rolling direction. The Pb concentration of the specimen 6 in the surface portion up to a depth of 0.2 μm from the uppermost surface was 120 times the average Pb concentration.

As shown in Table 3, Specimens 1 to 6 according to the present invention did not produce a non-uniform pattern and streaks, exhibited excellent etching properties after an electrolytic treatment, and had uniform etch pits over the entire surface.

On the other hand, Specimen 7 could not be sufficiently surface-roughened during an electrolytic treatment due to low Mg content. Specimen 8 could not ensure pit uniformity during an electrolytic treatment due to high Mg content.

Since Specimen 9 was produced with a long holding time from the completion of rough hot rolling to finish hot rolling, coarse recrystallized grains were partially produced due to the growth of recrystallized grains, whereby minute recrystallized grains could not be obtained at the time of completion of hot rolling. Since Specimen 10 was produced with a short holding time from the completion of rough hot rolling to finish hot rolling, a uniform recrystallized structure could not be obtained in the surface portion of the sheet material due to insufficient recrystallization. As a result, a non-uniform pattern and streaks were observed.

Specimen 11 produced a non-uniform pattern and streaks since the finish temperature of finish hot rolling was low and non-recrystallized portions occurred due to insufficient recrystallization. Specimen 11 also showed pit non-uniformity during an electrolytic treatment. Since Specimen 12 was homogenized at a low temperature, precipitation of Fe and Si was insufficient, whereby the pit pattern formed during an electrolytic treatment was non-uniform, and an unetched area was observed. 

1. A method of producing an aluminum alloy sheet for a lithographic printing plate comprising homogenizing an ingot of an aluminum alloy at 500 to 610° C. for one hour or more, the aluminum alloy comprising 0.05 to 1.5% (mass %; hereinafter the same) of Mg, 0.1 to 0.7% of Fe, 0.03 to 0.15% of Si, 0.0001 to 0.10% of Cu, and 0.0001 to 0.1% of Ti, with the balance being aluminum and unavoidable impurities, subjecting the homogenized product to rough hot rolling, a start temperature of the rough hot rolling being 430 to 500° C. and a finish temperature of the rough hot rolling being 400° C. or more, holding the product subjected to the rough hot rolling for 60 to 300 seconds after completion of the rough hot rolling to recrystallize the surface of the product, subjecting the resulting product to finish hot rolling that is finished at 320 to 370° C., and winding up the resulting product in the shape of a coil to obtain a hot-rolled product having a surface with an average recrystallized grain size in a direction perpendicular to a rolling direction of 50 μm or less.
 2. The method according to claim 1, wherein the aluminum alloy further comprises 2 to 30 ppm of Pb.
 3. The method according to claim 1, the method comprising subjecting the hot-rolled product to only cold rolling to obtain a sheet material having a specific thickness. 